The present invention relates to a mold apparatus suitable for injection-molding an article having a hollow structure, and a process for injection-molding an article having a hollow structure with the mold apparatus. Specifically, it relates to a mold apparatus which can effectively prevent "jetting" (also called "snake flow") in a molten resin injected into a cavity of a mold and is suitable for molding an article having an excellent appearance and a desired hollow structure therein, and a process for injection-molding such an article having a hollow structure with the mold apparatus. More specifically, it relates to a mold apparatus suitable for molding an article having a hollow structure which works, for example, as a path for the flow of a fluid, the hollow structure being a T-letter shaped manifold structure, a cross or "+" shaped manifold structure or a manifold structure for distributing a fluid in multi-directions, and a process for injection-molding an article having such a hollow structure with the mold apparatus.
For producing a molded article of a synthetic resin having a desired hollow structure, an injection molding method has recently increasingly come into wide use in which a pressurized fluid is introduced into a molten resin injected into a cavity of a mold to form a molded article having a hollow structure, since the so-formed article is almost free of a sink mark and distortion. This conventional injection molding method requires the use of a mold and an apparatus for introducing a pressurized fluid. Further, the mold is required to have a sprue portion and a runner portion for flowing a molten resin from a heating cylinder head of an injection molding machine to the cavity of the mold, and a gate portion is provided between the runner portion and the cavity.
An article having a manifold structure having a hollow structure which works as a path for the flow of a fluid is used, for example, as a tubing member, in various fields. An article of this type is used, for example, as a cooling tubing member for an engine, an intake manifold member for an engine, an exhaust gas tubing member or a hydraulic tubing member for a braking device in the fields of automobiles and motorcycles. Further, an article of the above type is used, for example, as a coolant tubing member for a refrigerator or a fluid-controlled actuator tubing member in the fields of electric appliances and machines. It is also used, for example, as a tubing member for a hot-water heater, a tubing member for a boiler, a tubing member for water supply or a tubing member for gas supply in the fields of construction and construction materials, or as a tubing member for a medical apparatus for a circulatory system or a tubing member for a medical apparatus for a respiratory system in the field of medicine.
Generally, the material forming an article having a manifold structure is a metal or a synthetic resin. When an article having a manifold structure is produced, for example, from a metal, a plurality of parts are assembled and welded. When an article having a manifold structure is produced, for example, from a synthetic resin, a plurality of parts are assembled, bonded and welded. For producing an article having a manifold structure by an injection molding method, there is known a method in which a core for forming a hollow structure is placed in a cavity of a mold and a molten resin is injected into a space formed by the cavity surface and the surface of the core. However, the problem of this method is that the mold structure is complicated. Further, there is known another method of forming a manifold structure by an injection molding method, in which a core of a water-soluble resin or a metal for forming a hollow structure is placed in a cavity of a mold, a molten resin is injected into a space formed by the cavity surface and the surface of the core, and then the core is dissolved in water or is removed by melting it. In this method, however, it is required to prepare a core of a water-soluble resin or a metal in advance, and it is required to remove the core after an article is formed, so that the production process is complicated. As explained above, in all the above-explained methods, a number of parts are required, the production process is complicated, an assembly step is required, and it is difficult to reduce the production cost. Further, the method of producing a manifold by the injection molding method with the core involves a problem of weld.
JP-A-6-262636 discloses a method of injection-molding a pipe-shaped article having a plurality of branches. In this injection molding method, no complicated mold structure is required. While a molten resin is injected into a cavity of a mold for producing a branched pipe, a pressurized gas is introduced through branch ends to form a pipe-shaped article. This injection molding method is, in principle, is a method for improving the appearance of an article (overcoming a sink mark and distortion) by introducing a pressurized gas into a molten resin, or a so-called gas-assisted injection molding method.
When an attempt is made to produce a molded article of a synthetic resin having an excellent appearance by a conventional injection molding method, and when the area of the cross-section of the cavity of a mold, which is perpendicular to the flow of a molten resin, is largely different in size from the area of the cross section of a gate portion, jetting takes place in a molten resin injected into the cavity, and the appearance and function of the molded article is greatly impaired to decrease the product value of the article.
For obtaining a molded article of a synthetic resin having improved physical properties such as high strength, high impact strength, high fatigue resistance, high creep resistance, high heat resistance, high hot water resistance, high oil resistance, high chemical resistance, etc., it is required to use a synthetic resin of high melt-viscosity such as a synthetic resin of having high molecular weight, a crosslinkable synthetic resin or a synthetic resin alloy. However, the problem of the use of the above resins is that jetting is liable to occur, so that it is difficult to obtain a desired molded article having an excellent appearance.
The method of introducing a pressurized fluid into a molten resin injected into a cavity of a mold is largely classified into a so-called full shot method in which a pressurized fluid is introduced after the cavity is completely filled with a molten resin and a so-called short shot method in which a pressurized fluid is introduced while the cavity is not completely filled with a molten resin. In particular, the short shot method has a problem in that when a spiral (coil-like) jetting occurs, it is difficult to introduce a pressurized fluid into a molten resin (in an amount insufficient for completely filling the cavity) injected into the cavity, and the pressurized fluid sometimes blows off the molten resin in the cavity so that no desired molded article is obtained.
For preventing the occurrence of jetting, there has been proposed a method in which the cross-sectional area of a gate portion is increased, or the injection rate of a molten resin is decreased, or the temperature of a molten resin is increased, or a resin having a low melt viscosity is selected. However, when the cross-sectional area of a gate portion is increased, it requires an additional cost to post-treat the gate-corresponding portion of the article. When the injection rate is decreased, the molding cycle is long so that the production cost increases. When the temperature of a molten resin is increased, the heat stability of the molten resin is impaired and the molten resin generates a gas or the resin has a decreased molecular weight so that the molded article has a poor appearance or decreased properties. Further, the method of selecting a resin having a low melt viscosity is not an essential solution for obtaining a molded article having high impact strength and high fatigue resistance, since, for these purposes, it is required to use a resin having a high melt viscosity such as a synthetic resin of high molecular weight, a crosslinkable synthetic resin or a synthetic resin alloy.
The injection molding method disclosed in JP-A-6-262636 is very effective for producing a pipe-like article having a plurality of branches. However, a pressurized gas is sometimes not properly introduced into a molten resin, and a molded article having a desired hollow structure is not always obtained. This phenomenon is liable to take place when a synthetic resin having a high melt viscosity or a synthetic resin reinforced with a reinforcing material such as a glass fiber is used. For obtaining a molded article of a synthetic resin having improved physical properties such as high strength, high impact strength, high fatigue resistance, high creep resistance, high heat resistance, high hot water resistance, high oil resistance, high chemical resistance, etc., it is required to select a resin from synthetic resins having a high melt viscosity such as a synthetic resin of high molecular weight, a crosslinkable synthetic resin, resins prepared by incorporating a reinforcing material into these resins, and a synthetic resin alloy. However, when the above resins are used, it is sometimes difficult to obtain a molded article having a desired hollow structure.